1. Technical Field
The present invention is directed towards corrosion inhibitors. More specifically, the present invention is directed towards sulfur based corrosion inhibitors for use in metal corrosion inhibition, particularly yellow metal.
2. Background Information
Copper corrosion inhibitors are widely considered a staple ingredient in most water treatment formulations. These inhibitors are designed to protect against the corrosion of the copper alloy surfaces found within industrial cooling systems, especially at the heat exchange surface. The accelerated corrosion of these surfaces and resulting galvanic deposition of copper onto existing ferrous metal surfaces can have detrimental effects on the structural integrity and operation of the cooling system. As a result, copper corrosion inhibitors have always been a staple ingredient in most water treatment formulations.
For at least the last thirty years, benzotriazole (‘BTA’) and its derivatives have dominated industrial yellow metal corrosion inhibitors. Its derivatives include tolyltriazole (‘TTA’) and 2-mercapto benzotriazole (‘MBT’). Their structures are illustrated as follows
By far, the most popular of these has been 4-5 methyl benzotriazole, or TTA. It has become the industry standard and is usually the only copper corrosion inhibitor considered by water treatment experts. Triazole inhibitors are typically dosed into cooling towers at a range of 2.0 to 5.0 mg/L. In closed loop recirculating systems, their dosages can reach as high as 25 to 50 mg/L.
Even though they dominate all other competitors, triazoles' dominance, including TTA, still have their weaknesses in certain applications. For example, tests have shown that chlorine added as a biocide can penetrate the thin tolyltriazole film causing accelerated corrosion rates. The tenacious, hydrophobic film formed with tolyltriazole makes it very resistant to breakdown in aqueous environments. However, because of the film's thinness, it is not very forgiving if breakdown does occur. At elevated levels, both chlorine and bromine have been found to attack and breakdown the formed film, causing corrosion inhibition failure. Therefore, a user must assure that there is residual inhibitor present in these situations to repair the damage.
Both BTA and TTA are believed to utilize the triazole functional group as their binding site to the metal, resulting in a protective film on the copper surface. Spectroscopic analyses have shown that the film formed is a 1:1 molar complex of Cu(I) and triazole. This complex is thought to stabilize Cu(I), preventing the copper from oxidizing further, thereby preventing the anodic reactions. The retardation of the cathodic reaction is believed to be accomplished by the hydrophobic backbone of the formed film, which inhibits the transport of hydrated, electronically active species to the metal surface. However, the properties of these two films are quite different. The film formed by TTA has been found to be more resistant to breakdown in aqueous environments. The methyl group on the TTA molecule is believed to sterically hinder the film's thickness, as well as offer more hydrophobicity. Both of these properties contribute to its greater resistance. However, as noted above, TTA's thin film is not as forgiving as BTA should breakdown occurs. In contrast to TTA, the BTA film is much thicker, consisting of many layers. Although it is more easily penetrated than the TTA film, its extra thickness helps act as a buffer against complete breakdown.
One of the most frequently claimed weakness of triazoles has been their susceptibility to degradation from oxidizing, halogenated biocides. This degradation is believed to affect both the formed triazole film and the residual inhibitor in solution, which has the potential to consume all of the added biocide. Most studies have indicated that free triazole, in solution, is susceptible to degradation in the presence of halogenated biocides. However, studies have differed on the degree of this degradation, ranging from severe and detrimental to mild and insignificant. There is even greater debate over the effect halogenated biocides may have on previously formed triazole films.
Some studies have proposed that the film is not damaged at all, but simply penetrated by chlorine. This attack is more pronounced immediately after chlorine addition, when chlorine concentrations are at their highest. Once the chlorine concentration falls, the corrosion rates fall back to baseline values. The more hydrophobic TTA film is more resistant to this type of low level attack than BTA, requiring more free chlorine to initiate attack. This penetration attack has been found with short term dosages of less than 1.0 ppm chlorine. Longer exposure times and higher concentrations have been found to damage the film in situations where no residual inhibitor is present. The hydrophobicity of the film does not seem to offer any added benefit against this type of attack. In contrast, bromine has been found to be much less aggressive to the metal because its larger sized atom cannot penetrate the TTA film.
To overcome triazoles' weaknesses, most water treatment experts recommend keeping a residual amount of inhibitor present in the water to repair any damaged areas of the film. It has become common practice in most traditional cooling water treatment programs to always maintain a constant residual level of triazole in the cooling water of around 2.5 mg/L active product. It is also advised to use a scheduled intermittent feed of inhibitor that occurs just prior to and also during any halogen addition. However, the most common reason for keeping a residual in the water, whether in combination with halogenated biocides or not, is to offer an additional level of security in case of film breakdown.
More recent tests have demonstrated that this need to maintain a residual amount of inhibitor such as azole may be more critical than previously suggested. These tests found that both BTA and TTA films are surprisingly weak, even when not in the presence of oxidizing biocides, breaking down immediately when no residual inhibitor is present. The need to maintain a residual amount of triazole in the cooling water is critical to the triazoles' success at corrosion inhibition. Without the residual inhibitor, the films offer very little sustained protection from corrosion. These findings demonstrate that the success of the azoles' corrosion protection relies solely on the immediate repair of damaged film by free inhibitor in the water, not in the formation of a tenacious, hydrophobic film. Still, there is room for improvement.
Various attempts have been made to develop viable alternatives to TTA in the last few years. These compounds have consisted primarily of triazole derivatives having more hydrophobic backbones that offer better resistance to halogenated biocide degradation. These past studies have focused on the degradation of the residual inhibitor in solution with very little discussion of the film's susceptibility itself.
Accordingly, there is a need for an alternative yellow metal corrosion inhibitor that overcomes the film susceptibility of triazoles, particularly with respect to chlorine. Further, there is a need for an alternative yellow metal corrosion inhibitor that provides improved resistance to degradation by biocides.